The use of pigments in paints, varnishes, printing inks, building materials, rubbers, etc. is well known. Organic as well as inorganic compounds have been widely used as colouring materials. Usually a pigment consists of (very) small particles which are practically insoluble in the applied medium, this in contrast to dyes, which are soluble in the applied medium.
The main white inorganic pigment is titania (or titanium dioxide). Two processes are used to prepare titania on a commercial scale. See for instance Ullmann's Encyclopedia of Industrial Chemistry, Fifth edition, Vol. A20, pages 271-281.
In the “chloride process” titanium containing raw materials as ilmenite, leucoxene, natural and synthetic rutile, titanium slag and anatase are chlorinated at 700-1200° C. Titanium tetra chloride is separated from the other chlorides by distillation. The titanium tetra chloride, optionally after further purification, is burnt with an oxygen containing gas at temperatures between 900 and 1400° C. to form titania. The pigment obtained in this way is very pure. Depending on the type of application some further treatment may be necessary.
In the other process, the “sulphate process”, the titanium raw material, especially ilmenite and titanium slag, are dissolved in concentrated sulphuric acid at 150-220° C. Removal of insolubles and precipitation of iron sulphates results in a concentrated titanyl sulphate solution. Relatively pure titania dihydrate (also called “mesa titanic acid”) is precipitated by hydrolysis of the sulphate solution at about 100° C. The remaining impurities, especially metal sulphates, are largely removed in further purification stages, especially by washing with diluted acid and by bleaching. The hydrate is filtered (usually using a rotary vacuum filter) until a titania (anhydrous TiO2) content of 30-40 wt % is obtained, calcined, ground and further treated, depending on the type of application. Calcination (suitably at a temperature below 1000° C., especially between 400 and 700° C., 0.1 to 3 hours) is usually carried out in rotary kilns. Approximately two-thirds of the residence time (often 7-20 h in total) is needed to dry the material. Above 500° C., sulphur trioxide is driven off which partially decomposes to sulphur dioxide and oxygen at higher temperatures. In another form of the process the obtained titania hydrate is suspended in water and spray dried, optionally followed by calcination.
Beside the use of titania as a pigment, there are also other applications. One other application of titania is the use as catalyst carrier. A problem in the use of titania as catalyst carrier, especially with titania produced in the sulphate process, is the high content of sulphur, especially sulphate, in the titania. The presence of sulphur, either as adsorbed sulphuric acid or in the form of metal sulphates or in any other form, is often detrimental to the performance of the catalyst prepared from the titania carrier. The activity, the selectivity and/or the stability the may be negatively affected. In a number of cases the regeneration by means of hydrogen treatment results in the formation of hydrogensulphide, which negatively affects catalyst performance.